1. Technical Field
The present disclosure relates to an infrared sensor on which a phononic crystal structure is mounted.
2. Description of the Related Art
In conventional thermal infrared sensors, a structure has been proposed where an infrared receiver is disposed with a space from a base substrate via a beam, as described in Patent Literature 1. This structure aims at insulating the infrared receiver from the base substrate thermally, and infrared receiver sensitivity improves as thermal insulation performance of the beam increases. As described in Patent Literature 1, using porous materials such as aerogel as a beam allows improvement in thermal insulation performance. However, a simple porous structure without order only leads to reduction in thermal conductance as porosity increases, providing limited thermal insulation performance.
Meanwhile, Non-Patent Literature 1 or Patent Literature 2 discloses that introduction of through holes or pillar-shaped resonators that form periodic lines on the order of nanometers (in a range from 1 nm to 1000 nm) into a thin-film substance allows reduction in thermal conductivity of a base material that constitutes the thin film. Such a substance is referred to as a phononic crystal. Because of the reduction in thermal conductivity itself of the constituent material, this provides a thermal insulation effect greater than reduction in thermal conductance resulting from introduction of porosity as compared with the simple porous structure.
The following describes a mechanism by which the phononic crystal controls thermal conduction. In an insulator or semiconductor, heat is mainly carried by lattice vibration called phonon. Dispersion relation of phonon (relation between frequency and wave number, or band structure) is determined for each material. Thermal conductivity of an insulator or semiconductor is determined by phonon dispersion relation. In particular, a heat-carrying phonon ranges in a wide frequency band from 100 GHz to 10 THz, and the phonon corresponding to this band determines a thermal conduction characteristic of the material. The frequency band of the heat-carrying phonon is defined here as a heat band. In a phononic crystal, introduction of a periodic structure allows artificial control of original phonon dispersion of the material, allowing control of thermal conductivity itself of the material. In particular, examples of variation that affects thermal insulation performance in a dispersion curve include formation of a phononic band gap (PBG). When the PBG can be formed in the heat band, phonons inside the PBG cannot exist and will not contribute to thermal conduction. As a result, thermal conductivity can be reduced.
Introduction of such a phononic crystal structure into the beam of the infrared receiver allows improvement in sensitivity of the infrared sensor.